You need young, small and green walnuts. Peel them until you get to the white flesh and soak them in 0.5% solution of citric acid or 2-3 teaspoons vinegar. This prevents them from browning. Put them to boil in some water, change it and then put them to boil again.
Repeat this process 3-4 times, boil the walnuts a few minutes until soft. Then soak them in cold water for 10 hours. Make a syrup of 2 lb (1 kg) sugar and 2 cups of water and bring it to a boil. Add the drained nuts to it and boil the jam on moderately high heat until you get the right density.
The jam needs to be constantly stirred and the foam removed. Once you remove it from the heat, add 1 1/2 tsp citric acid.
Distribute it into small jars, close them well and store in a cool, dark place.